Black Oath
by Rielin
Summary: He promised himself, to avenge his family and to find the mysterious woman― only to betray the human race and seal the contract of Black Oath.
1. Chapter 1

**bgm** 《 Haikyuu! OST - Team Potential Extended 》

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 **Black Oath**  
 _"Corrupt my mortals, I will give you my soul  
―the contract of black oath,"_

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The boy sprinted with heavy pants as he scurried through pieces of broken edifice plastered with fresh blood. The cries of many villagers echoed loudly into his ear although he tried his best to ignore them. Innocent tears were constantly forming within his midnight blue colored eyes, and hopelessly they fell upon the villagers that are now in an eternal sleep, after experiencing the devastation. The boy glanced behind, only to take a glimpse of devilish horns and black fire burning through the intricate village once called home. As his gazed was focused on the background behind him, the boy clumsily tripped on fallen debris and helplessly fell upon. He raised himself up with the little force he has, only to have his palms painted with blood of his father. How desperately he wanted to cry in plea, while embracing his lifeless father within his arms, yet he had no time to lose. With two knees that has been scraped along the cobblestone, he ran and ran; until he faced a dead end. He glanced around with panic and found thrown away boxes along with trashes that were laying across the ground lazily. He swiftly grabbed the items near him and covered himself, the darkness of fear overwhelming his soul as he shivered in angst.

The boy's heart was pounding rather furiously and beads of sweat was trickling down the side of his face in nervousness. He clenched his fist so tightly that his own nails were digging through his skin, blood slowly forming within his fingertips which was soon absorbed by his nails. He constantly prayed, for his family to be alive, for his friends to be alive, for his village to be alive. Yet the devastating sight of his dead father flew across his mind within that split second, and the boy couldn't do anything except to let out silent desperation.

How long has it been? He thought, as he continued to shiver in fear as he closed his eyes in order to block the corruptions. Even before he could reanalyze his own thoughts, a hint of soft warmth embraced his cheek. As the warmth he wished for, the warmth he prayed for tickled his own skin, the boy slowly fluttered his eyes open. He glimpsed a silky, smooth white hair fluttering in the dusty wind, along with crystal blue eyes that shined in the void of darkness. He glanced up as he constantly blinked in order to clear his blurry vision, yet it was no use. Clumps of blood and dust have been collected on top of his eyelashes, clouding his eyesight as he desperately tried to clear his focus of the woman. She did not say anything, she did not show any emotion; the only expression the boy could acknowledge was the warmth of her hand as if was reassuring him. The boy closed his eyes, for how tired he was, and slowly the tenderness of her hands faded him to a peaceful sleep.

* * *

Sunshine peeked its way through the obstacles and made its ways on top of dark blue colored eyelashes, which was scrunched up due to sudden brightness. The midnight blue eyes that were covered by the clouds of eyelashes slowly revealed itself.

Ciel glanced about, then quickly came to a realization that he's currently cuddled up in a sleeping bag, along with other members of the army that are in the same situation compared to Ciel. He let out a silent sigh and removed himself from the bag and folded it rather neatly and walked out of the room. The fresh air of crack of dawn welcomed the fair male as he inhaled deeply. With a deep exhale, he reflected back to just a mere year ago.

His entire family, no― his entire village have been swept by the devilish claws of the demons. The desperate plea of his friends and the sight of his dead father haunts him to every night as he always awake from nightmares, yet the ambitious feelings of revenge grows bigger and bigger within his soul. Then he remembers the mysterious woman with silky white hair, and with crystal clear eyes that seemed to shine its beauty past through the devastation. Who was she? He would wonder often times, yet no answers could be found. Years after the bloody event, he hadn't spotted a single sight of her, but he knew, that she was not a mere illusion and that she was real, and she was the one that had helped him stand on his feet once again. Even he, does not know her exact existence. Is she human, or devil? Why was she there― in middle of a battle field? Those two were his main goals; to avenge and to find her. In order to achieve both of his goals, he knew for a fact that he had to grow stronger; joining the military that fights the demons was the only solution.

"Ready up, cadets,"

With those words, the young trainees in Demon Military Army raised their awarenesses and brushed off their sleepless exhaustion. Ciel grasped onto his dual gunblades and buckled them onto his side belt as he walked out of the room, as he always expects to fight a demon or to meet the woman; he always has to be alarmed and careful, not to make the tiniest mistake that will lose his chance of fulfilling his goals. He glanced down toward his hips, and the sun that has just rised shined its ways and reflected the true light of his dual gunblades. Gun and a blade, indeed it was a powerful weapon and that he is blessed with.

Before Ciel left on an embark to join the Demon Military Army, he sprinted along the fallen debris and dead bodies. For a young boy who had just experienced traumatic fatalities of his loved ones, Ciel is considered to be courageous. Although he cried multiple shatters of tear, his screams did not echo throughout, instead, he would swallow the pain away. After passing the soulless body of his mother lying on the floor, he reached the basement of his house. Inside is the weapon that his father had kept away for years― he never knew why; perhaps it was for days like this. The dual gunblades, he picked up and with the weapon that contained the souls of his families, he seek avenge.

"Arms up,"

With the general's alarming words, Ciel came back to his realization that he has entered the ancient battle ground. The intricate designs of the edifices are now piled upon each other, with dusts that were collected along with overgrown plants embracing the broken puzzles that completes a building. From afar, Ciel heard hint of crows bickering, ready to snatch up any source of life within this ancient ground― but of course, not a single plant has bloomed within the bloody battlefield. He inspected further as he kept pace with rest of the army, yet somehow, this ancient ground seemed familiar and nostalgic. He caught a glimpse of a broken piece of door, which was painted in ivory and had intricate design. Then he saw a window, painted rich brown with two handles placed on the inside. Then the mixture of various gray colored cobblestone that rang loud and clear whenever a foot knocked on its surface. The copper colored roof that must have created a safe shelter for the villagers that once lived here. Then all of the puzzle pieces he had just glimpsed combined with one another to create the final picture― this ancient ground is his hometown. The place where he was given birth, the place where he created and defined his own morals, the place where he was part of a society, and the place where he said his last farewell.

"Cadet, don't make us wait,"

The general's voice echoed into Ciel's ear as he realized that he had stopped his movement and was staring off into the ancient village while reminiscing his past. Ciel quickly bowed his head as apology and fastened his footsteps to catch up with rest of the army. The innocent memories of the past cannot interfere with his mission, for that will put him into a vulnerable situation and his goal will never be accomplished.

"Demons,"

The general said with a rather quiet, yet firm voice. Acknowledging his warning, the entire group of army glanced about as they perked their ears up in order to detect any signs of demons lurking from behind. Indeed, the general was sharp. Ciel could feel a dark aura lingering in the air, not too far but not too close. The sound of sharp, but faint footsteps could be heard if one listened carefully, and the shadows of the demons swept back and forth behind the dark alleys.

At that moment, the general yelled firmly―

"Attack!"

Placing one of his dual gun blade in the front and the other in the back, Ciel prepared for any sudden attacks by the demons. At last, the first step of avenging his family is bound to be happen, the place where his family died, is the place where he will fight.

All of a sudden, a demon lunged from the behind and quickly, Ciel swiped his movements to the left to dodge the piercing claws of the demon. He jumped back and aimed both of his dual gunblades and shot out his first two bullets, cleanly killing the first attacker. He let out a quick sigh, the ancient battle ground will soon be plastered with blood once again― but this time, it will be the blood of the demons. Smile of triumph could be found under lips of the fighter and quickly he prepared for another attack ready to be lunged. Then at that moment, from the corners of his eyes, he saw a hint of long, silky white hair fluttering in the air. Without hesitation, he shifted his head with quick movements, but the sign of the white hair could not be found.

"At last… I've found you,"

He whispered.

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 **note:**

i've never lu x ciel fanfiction and i don't know that much about them  
so this was totally out of my comfort zone lol.  
this is diabla (alter of evil state) x demonio, and obv the white hair is lu.  
the story revolves around ciel and lu will barely appear lol oops.

i planned this story to be 3-4 chapters, but that didn't work out  
so it will only be 2 chapters long.

hope you guys enjoyed, don't forget to favorite/follow/review!


	2. Chapter 2

**bgm** 《 Haikyuu! OST - Greed 》

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 **Black Oath**  
 _"Corrupt my mortals, I will give you my soul  
―the contract of black oath,"_

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Alley after another, the white hair swayed in the air as if the mysterious woman was leading Ciel to somewhere unknown. Ciel chased after, ignoring his teammates yelling for him, however he could not easily ignore the demons that were constantly lunging on the fighter. Grunting with irritation and desperation of losing the sight of the woman he had been searching for years, Ciel sliced the bodies of the demons that were jumping for an attack towards the human. His breath became heavy as he used more and more energy on desperately running and fighting the demons off, and swiftly, he was becoming exhausted. Due to extreme tiredness, Ciel was not able to detect the existence of a demon that successfully landed a piercing attack on Ciel's back leg, its sharp claws jabbed into his skin as blood spurted out. Grunting with pain, Ciel stabbed the demon's hand as it yelled in agony. The demon's hand was jabbed into the edge of Ciel's dual bladegun, and as it hung loosely in the air, Ciel quickly shot another bullet as the demon busted with torment, rain of black blood splattering on Ciel's uniform.

Ciel walked in a jerky motion due to discomfort of pain in his back leg. With heavy breaths, he quickly hid his shadows in a dark alley as he tried to ease the pain, yet the blood was profusely gushing out of his skin. Beads of sweat were forming on top of his forehead which quickly rolled down his temples, only to be fallen on top of the cold cobblestone.

He couldn't dare himself to be let down like this; after a simple attack of the demons, he is already exhausted. How would he avenge his family's death in this condition? Then the thought of the mysterious woman fluttered back into his mind. The woman who reassured him as he was in an internal torture; she was the only one who offered a hand to the hopeless child and helped him to get back on his feet. He doesn't even know her name, her race, her voice― nothing. He could not define her.

 _She was just there… only couple feet away,_

He thought to himself as he grunted with pain, the pain in his leg aching more as he shifted the position.

 _I need to… see her,_

He does not know what he will achieve after finding her; meeting her once again was just a simple, pure mission he wanted to accomplish. However, even if he do find her, will she disappear once again like that night in the past? Or would he have the chance to talk to her, will he ever hear her voice? Thoughts about the mysterious woman constantly lingered in his mind, almost impossible to even erase her being trapped inside his soul. Effectively, the injured soldier's eyelids became heavier, as his breaths started to soften. Slowly, the tensed shoulders started to loose, and his visions started to blur― into the void sleep, he entered.

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Pitch black. Ciel glanced about as panic started to rise between his fingertips, causing his hands to tremble in hint of fear. He couldn't see anything, or perhaps there wasn't anything. Either the lights were turned off, or there was no source of light.

"Hello?!"

He yelled out rather courageously, only to have his voice return as an echo. The male started to walk, then he started to run; even though he had no idea where he was going nor the direction he was running to.

Then at that moment, Ciel caught a glimpse of light from afar. With hopes growing inside his soul, he started to dash after it, not blinking his eyes even for one second in hopes of trying to not lose the sight of the light. As he approached the mysterious radiance, he realized it was a rich blue colored fire. There was no source of candles, wood, or anything, yet the fire was lit up rather brightly, flowing in the air. He reached out without thinking; it was then Ciel came to another realization, that the blue fire did not burn him, instead it wasn't even hot. Perhaps he lost his sensation of touch, yet he could feel his droplet of sweat trickling down his spine. He knows for a fact that blue flames contain more heat than red flames, therefore this was unusual, yet he was somewhat mesmerized by it. He had never thought fire could hold such beauty. In fact, he was traumatized by the walls of fire that had burnt down his past life, yet this fire was different Ciel tried to grasp the flames in between his fingers, he would place the palm of his hands under as if he was the one conjuring it, yet nothing worked to actually move the fire.

Then a thought struck him.

 _Is this a spell? A demon's spell?_

Sudden panic started to rise within his alarmed soul, the fingers that were trying to attach itself to the fire started to tremble in fear. The midnight colored pupils of his eyes started to shake in nervousness. Ciel let out a silent sigh to relieve his alarmed soul, and cautiously he reached out for his dual gun blades that were attached to his belt located next to his hips. He slowly raised his weapons, one on the front and one on the back as his ears stayed perked up for any signals of demons appearing. However, although minutes have passed, there was no signs of demons. Not a single sound, not a single sight, and not a single smell. There was no demons around him. He let out a relief as he loosened his tensed muscles.

 _...At last, I've found her,_

Ciel gently closed his eyes for relaxation and as he slowly opened them, the sight of the woman was in front of him. Just couple feet away, and in between them was the blue flame. He reached his hand out as he attempted to call her, yet he realized he doesn't even know her name.

"My name is Lu,"

Her angelic voice echoed throughout the void, resonating back into Ciel's mind as if she read his thoughts. Surprisingly, he wasn't confused or nervous. He was just content enough to finally meet her, even if this was a dream. Ciel gaped his lips in order to speak, yet no voice rang out; he was muted.

Slowly, the female's back that was facing Ciel slowly turned, her white hair that seemed to glow swayed in the air elegantly with each movement. As she faced Ciel, he couldn't do anything except to gape in shock. She had no face. The pale skin of hers just radiantly glowed; the crystal blue eyes, her petite nose, and her cherry-colored lips missing. Perhaps, this is a dream. Because Ciel cannot fully recall her physical appearances, his mind decided to rather erase her face. But how was he able to hear her voice? Years ago, when he had faced her in actual life, he wasn't able to hear her voice, nor acknowledge her name. After reanalyzing his thoughts, slight disappointment rose within Ciel, he wasn't able to figure out if this was just a mere dream, or reality of a demon's curse.

"Promise me a black oath…"

She softly whispered into Ciel's ear, gently brushing his midnight colored hair, tucking his long strands behind his ear.

"..."

She whispered something once again into his ear, yet it came out as a blur. He tried to focus his eyes onto her face, yet she was slowly disappearing. As confused as he was, he reached his hand out in order to stop her from disappearing, yet it was too late. Then the blue flame that was lighting itself slowly devoured her, burning her into nothing but voidness.

"Lu!"

He was finally able to speak, his desperation echoing into the blackness, unable to be heard by anyone.

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With heavy breaths and sweaty palms, Ciel's eyes suddenly opened, releasing the feared panic that was trapped inside his eyelids.

 _A dream._

He wasn't able to tell if he was glad that it was a dream, or if he wasn't glad that it was a dream. He was able to see her, yet he wasn't.

From afar, he could make the sounds of guns being fired, swords clashing one another, and the sharp claws of demon ripping through the flesh of humans. Then the pain that was lingering in his back legs started to ache. He was back to reality.

At that moment, he heard a rustle from not too far, perhaps just mere couple feet away. Cautiously and silently, he raised himself up although this injured leg jerked in pain. He prepared himself for a sudden lunge as he placed his weapons before him. The demon was near. If Ciel can time the attack right, then as soon as the demon is about to pass next to the alley Ciel is staying at, he will be able to lunge onto the demon.

Closer and closer, the sharp claws of the demons echoed throughout the brick stones. Ciel held his breath, did not move an inch, until it was the right timing.

 _Now!_

He turned his back then jumped a foot away from the demon, appearing himself as he aimed both of his gun blades toward the head of the demon.

Then he saw―

"I've found you,"

She whispered. The silky white hair that shined under the moonlight, the crystal cyan eyes that glistened with courage yet contained hint of happiness, and the same angelic voice that resonated into his dream. Her face wasn't a blur, her voice didn't come out as a mumble, and no blue flames devoured her. She was real. She was standing before his eyes like the night she had rescued him. He found the mysterious woman. He found the one whom he had loved for years. He realized, it was love.

Ciel's grip on his weapons started to strengthen itself.

 _She's a demon… I have to kill her,_

He thought, the grip becoming stronger and stronger, yet he was unable to fire the bullet.

 _But I love her._

He thought as he lowered his weapon back down.

Lu let out a smile, not a mischievous one, but true smile contained with nothing but happiness. She gaped her petite lips, and the angelic voice echoed into Ciel's voice once again.

"I've lost everything. Before the embers of my life go out, promise me a black oath,"

The voice that was inside his dreams and the voice that is in the reality slowly completed the puzzle, becoming one, full voice.

"Will you become my royal strength?"

She asked as the chilly night wind played with her long, silky strands. Her crystal eyes held nothing but plea. She held out her hand, waiting for him to accept, or waiting for him to kill her.

 _Kill her.  
_ _Accept her._

Ciel's eyes shaked in nervousness, what was the right choice? Before he could give another thought ―

He accepted her hand.

White aura started to soared from the ground and gleamed before their feet. Blue flames started to make their way up toward the top of their head as the contract of Black Oath was activated.

He wasn't able to fulfill his mission. He wasn't able to avenge his family. He betrayed the human race. Instead, he had chose to fulfill the other, to go with his beloved. He had two questions but only had one answer. Perhaps this was the punishment for falling in love with a demon, perhaps the answer was already made the day of the bloody battle.

"I'll give you my soul for you,"

He whispered and she smiled.

With nothing but betrayal that has corrupted his heart toward the humans, the male slowly turned into a demon.

 _Diala and Demonio―_  
 _The Altar of Evil and Transcendence―_

 **The contract of Black Oath is sealed.**

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 **note:**

turns out i used haikyuu ost for both chapters. ironically, i've never watched haikyuu but i love their ost XD  
honestly, this is my favorite out of all the stories i've written and i'm slightly disappointed that  
it's only 2 chapters long. :c i thought about adding maybe Lanox adventures in this,  
but it didn't flow with the story so i had to edit it out rip.

anyways, thank you guys for reading my first lu x ciel story!  
as always, please leave a review/favorite/follow as it will benefit me c:


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